For a motor having a brush, electricity is supplied through the brush slidably contacting with a commutator. A coil wound around a core of a rotor is connected to the commutator. When electricity is supplied to the coil, the rotor starts to rotate by virtue of the forces of attraction and repulsion applied from a permanent magnet provided in a housing so as to face the rotor.
In the motor having the configuration described above, when the motor is operated, the brush slides relative to the commutator, which is in contact with the brush. In such a situation, the slidable contacting surfaces of the brush and the commutator tend to wear out, a phenomenon that is liable to create problems. Conventionally, for purposes of restricting the degree of wear of a brush, materials used for making a brush have been varied, or the hardness of a brush has been controlled so as to restrict discharge of sparks occurring at the slidable contacting surfaces at a time of electrical or mechanical wear or when the motor is driven.
On the other hand, in the cases of a motor having a brush, intended for use in a vehicle, a metal-graphite brush, made by a process of sintering graphite particles and copper particles mixed in a binder, has been conventionally known (JP2001-298913A).
A conventional method for manufacturing a metal-graphite brush is as follows. First, natural graphite particles, as a base material, and a phenol resin solution, as a binder, are mixed. Next, a lubricant, such as molybdenum disulfide, is added to the mixture. Then, the mixture is sintered in a nitrogen-rich atmosphere at a temperature within a range of from 700 to 800° C. In this case, the film of dissolved phenol resin formed on the surface of the graphite particles is carbonized by a process of sintering, and thus becomes amorphous graphite. This amorphous graphite, as a binder, binds graphite particles. In addition, because a part of the organic substances, originally included in the solution of phenol resin, sublimate as a carbon dioxide, or as water vapor, many porosities are formed on both the surface and the interior of the metal-graphite brush. Thanks to the hygroscopic property of the graphite particles of which the metal-graphite brush is formed, the metal-graphite brush, manufactured according to the method described above, can absorb moisture existent in the atmosphere, as long as the metal-graphite brush is left in atmospheric air.
If the metal-graphite brush described above is employed in a motor, when the metal-graphite brush is operated, the temperatures of the slidable-contacting surfaces of the metal-graphite brush and of the commutator rise. Accordingly, moisture, originally contained in inner porosities located near the slidable contacting surfaces of the metal-graphite brush, starts to vaporize. Then, the vaporized moisture is provided between the slidable contacting surfaces of the metal-graphite brush and the commutator. Therefore, because a coefficient of sliding friction between the slidable contacting surfaces of the metal-graphite brush and the commutator is lowered, in other words, because of the effects of gaseous lubrication, the degree to which the metal-graphite brush wears can be reduced.
If the motor including the metal-graphite brush described above is utilized for a vehicle, because of the influence of heat generated by an engine accommodated in an engine room of the vehicle, the temperatures of the slidable contacting surfaces of the metal-graphite brush and of the commutator may on occasions rise to 100° C. or more during operation of the motor. In these circumstances, moisture that was originally absorbed by the porosities of the metal-graphite brush tends to vaporize at a significantly higher rate than the rate of vaporization that would occur at a room temperature. Accordingly, the motor begins operation in conditions where there is a lack of vapor between the slidable contacting surfaces of the metal-graphite brush and the commutator. Therefore, a coefficient of sliding friction between the slidable contacting surfaces of the metal-graphite brush and the commutator rises, a phenomenon that tends to lead easily to wearing of the metal-graphite brush.
Accordingly, when the conventional metal-graphite brush described above is utilized in conditions of high temperature, in contrast with circumstances where it is utilized at room temperature, the degree of wear relative to a unit operation time rises. As a result, longevity of the motor, including that of the metal-graphite brush described above, is adversely affected.
A need thus exists for a metal-graphite brush, which is hard to wear, and has long period of longevity, and a motor including such a metal-graphite brush. The present invention has been made in view of the above circumstances and provides such a metal-graphite brush and such a motor.